Yes, that “manifesto” is of questionable origin. The owner-admin of the 8chan site where it was posted says it was definitely not uploaded by the shooter; it came from another source or via an unrelated path. And if you read it (as I have), it certainly doesn’t jibe with the shooter’s posts on social media.
Yep. And it should be assumed as and dismissed as a fake before even being debated at any length at all.
re: “Yes, that “manifesto” is of questionable origin. The owner-admin of the 8chan site ...”
Overlooking a few things here chum, such as, the (purported) author of this piece is still LIVING.